Electric discharge devices

ABSTRACT

The inclusion of an alkali metal halide in the gas fill of a high pressure mercury and aluminium halide discharge lamp improves the color appearance of the lamp; notably by lowering the color temperature, while maintaining or even increasing the efficacy and stability of the discharge. Preferred contents are 10 to 30 mg cm -3  mercury, 5 to 100 μmol cm -3  aluminium halide and 0.5 to 150 μmol cm 3  alkali metal halide, with the molar ratio of alkali metal halide to aluminium halide in the range 0.1 to 1.5 and the total number of aluminium and alkali metal atoms preferably greater than the total number of halogen atoms.

The present invention relates to high-pressure metal halide electricaldischarge devices. In metal halide lamps the discharge takes place in agas atmosphere comprising a rare gas, mercury vapour and the halide ofone or more elements. The rare gas is present to make the dischargeeasier to start. At the operating temperature of the lamp, the mercurypressure is usually in the range of 0 to 30 atmospheres.

Metal halide discharges comprising aluminium chloride, mercury and arare gas (as described for example in British Pat. No. 1 190 833 and 1253 984) are known to have high efficacies and spectral powerdistributions that are continuous or quasi-continuous. They thereforehave desirable colour rendering properties. Other aluminium halidesafford similar spectra although with lower efficacies. The aluminiumhalide discharges suffer from having undesirable colour appearance,being either on the green side of white or having too high a colourtemperature. Their arcs also tend to be constricted and this causesinstability.

According to the present invention, halides of one or more alkali metals(Li, Na, K, Rb, .[.Ca.]. .Iadd.Cs.Iaddend.) are added to aluminiumhalide discharges in the presence of mercury and a rare gas. The alkalihalides introduced into the discharge filling modify the colourappearance, notably by lowering the colour temperature, whilstmaintaining and in some cases increasing the efficacy, increasing thestability and maintaining adequate colour rendering properties.

Typical practical embodiments of the present invention are metal halidedischarge lamps having a discharge vessel, usually made of fused silica,the wall of which is power loaded at 10-100 Wcm⁻², which contains a lowpartial pressure of rare gas, from 0 to 30 mg.cm⁻³ of mercury, analuminium halide and an alkali metal halide such that the ratio of thenumber of molecules of alkali metal halide to aluminium halide (assumedfor this purpose to be in the form of trihalide) is in the range 0.1 to1.5. Such embodiments normally contain sufficient aluminium and alkalimetal halide in the discharge vessel for unevaporated aluminium halideand alkali metal halide to be present in the vessel during the operationof the lamp so that the vapours of the halides in the discharge aresaturated. It is sometimes advantageous to add additional aluminiummetal to prolong electrode life (as described in British Pat. No. 1 253948).

The invention will be further described, by way of example withreference to the accompanying drawings, in which:

FIG. 1 is an emission spectrum of one example of a lamp according to theinvention containing sodium chloride and aluminium chloride;

FIG. 2 shows one type of discharge tube embodying the invention; and

FIG. 3 shows a second type of discharge tube embodying the inventionmounted in a suitable lamp form.

In a preferred form of the present invention, sodium chloride andaluminium chloride are used in the discharge lamp, which also containsmercury and rare gas. The total number of atoms of aluminium and sodiumis preferably larger than the total number of chlorine atoms, sincewhere the total number of metal atoms (aluminium and alkali metal) isgreater than the number of chlorine atoms electrode erosion is reduced.One effect of sodium additions can be seen in he spectrum shown in FIG.1 in which the sodium D lines are apparent. Similar spectra areobserved, for example, with sodium iodide and aluminium chloride or withsodium iodide and aluminium iodide discharges. A further effect is thatdischarge stability is increased and 50 Hz flicker is reduced tonegligible proportions.

The preferred ranges for the constituents are: mercury 10-30 mg cm⁻³,aluminium trihalide 5-100 μmol cm⁻³, alkali metal halide 0.5-150 μmolcm⁻³, with the preferred range for the ratio of the number of moleculesof alkali metal halide to the number of molecules of aluminium chlorideis 0.1 to 1.5 (all molecules being regarded as in the monomeric form).

By way of example only, the lamps illustrated in FIGS. 2 and 3 will nowbe described.

In the lamp shown in FIG. 2, a silica discharge tube 11 is closed ateither end by pinches 12 which make hermetic seals around current leads13. These current leads are connected to tungsten electrodes 14 whichmay incorporate an emitter material. The discharge takes place betweenthese electrodes. The discharge vessel has a length of 35 mm, aninternal diameter of 10 mm and a volume of 2 cm³. The electrodeseparation is 27 mm.

A similar discharge tube, shown in FIG. 3, has a volume 4.3 cm³,internal diameter 15 mm, length 35 mm and electrode separation 20 mm,and is mounted in an outer envelope 15 made from hard glass with a frameof a type that is known to inhibit migration of alkali atoms through thesilica. An auxiliary electrode 16 is provided for ease of starting. Inboth cases the temperature of the discharge tube ends may be increasedby coating with zirconia or by any other suitable means.

The dosing components for the discharge tubes of this invention may beintroduced into the tubes in any convenient form. By way of example, thehalogen may be dosed as a compound of mercury, e.g., Hg₂ Cl₂, ordirectly as the metal salt, e.g., AlCl₃, or separately as metal andhalogen, or in any other convenient way. Alkali metals may be doseddirectly, as the metal, or as halogen compounds e.g. NaCl, NaI, CaI.

EXAMPLE 1

The discharge space of a lamp as shown in FIG. 2 was dosed with

40.2 mg Hg₂ Cl₂

11.3 mg Hg

3.3 mg Al

2.3 mg NaCl

and filled with 20 torr of argon at room temperature. This lamp wasoperated at a power of 400 W when the light efficacy was 106 lm W⁻¹ andthe lamp had a warm colour appearance. The colour temperature of thislamp was 3100 K and the colour rendering obtained therewith wassatisfactory.

By way of comparison an aluminium chloride lamp with similar dosing butno sodium chloride has similar efficacy but a colour temperature of7,000 K and greenish white colour appearance but the same colourrendering index of 65.

EXAMPLE 2

The discharge space of a lamp as shown in FIG. 2 was dosed with

38.6 mg Hg₂ Cl₂

10.9 mg Hg

3.8 mg Al

10.2 mg NaI

At a power of 350 W, the efficacy was 100 lm W⁻¹. The colour temperatureof the lamp was 2,450 K and the lamp had a warm colour appearance.

EXAMPLE 3

A discharge tube similar to that shown in FIG. 2 but having a 6 mm bore,an electrode separation of 27 mm and a volume of 1 cm³ was dosed with

17.2 mg Hg₂ Cl₂

9.7 mg Hg

3.8 mg Al

0.7 mg NaCl

and filled with argon to 50 torr at room temperature. The lamp at 200 Wgave a light efficacy of 90-100 lm W⁻¹. The discharge was stable and hadno perceptible flicker.

EXAMPLE 4

A discharge tube have a volume of 0.65 cm³ with an electrode separationof 10 mm and a fill of

9.3 mg Hg₂ Cl₂

8.5 mg Hg

1.45 mg Al

0.52 mg NaCl

had a light efficacy of 70-80 lm W⁻¹ at 200 W and a colour temperatureof about 3,200 K, again with good stability and no perceptible flicker.

What we claim is: .[.1. A high-pressure electrical discharge lampcomprising: and alkali metal halides are present during operation of thelamp..].
 3. A discharge lamp as claimed in claim .[.2.]..Iadd.9.Iaddend., in which the total number of aluminium and alkalimetal atoms is greater than the total number of halogen atoms.
 4. Adischarge lamp according to claim .[.2.]. .Iadd.9 .Iaddend.wherein saidgas fill comprises mercry in the range 10 to 30 mg cm⁻³, aluminiumchloride or iodide in the range 5 to 100 μmol cm⁻³ and sodium chlorideor iodide in the range 0.5 to 150 μmol cm⁻³.
 5. A discharge lampaccording to claim 4 in which the total number of aluminium and alkalimetal atoms is greater than the total number of halogen atoms. .Iadd. 6.A high-pressure electrical discharge lamp comprising:alight-transmitting discharge vessel; spaced electrodes in said vessel;and a gas fill in said vessel comprising mercury vapour, rare gas,aluminum halide and at least one alkali metal halide, the molar ratio ofthe alkali metal halide to the aluminum halide varying from 0.1 to 1.5..Iaddend..Iadd.
 7. The lamp of claim 6 where the alkali metal halide isselected from the group of a sodium halide and lithium halide..Iaddend..Iadd.
 8. The lamp of claim 7 where the sodium halide is sodiumiodide. .Iaddend..Iadd.
 9. A high-pressure electrical discharge vesselcomprising: a light-transmitting discharge vessel having a wall powerloaded at between 10 and 100 Wcm⁻² during operation; spaced electrodesin said vessel; and a gas fill in said vessel comprising a rare gas,mercury in amount up to 30 mg cm⁻³, aluminum halide and an alkali metalhalide selected from the group of halides of lithium, sodium, potassium,rubidium, cesium and mixtures thereof, said alkali metal and aluminumhalides being present in such quantities that the molar ratio of alkalimetal halide to aluminum halide is from 0.1 to 1.5 and unevaporatedaluminum and alkali metal halides are present during the operation ofthe lamp. .Iaddend..Iadd.
 10. The lamp of claim 9 where the alkali metalhalide is selected from the group of a sodium halide and a lithiumhalide. .Iaddend..Iadd.
 11. The lamp of claim 10 where the sodium halideis sodium iodide. .Iaddend.